Design of an ultra-broadband microwave metamaterial absorber based on multilayer structures

An ultra-broadband double-layer microwave metamaterial absorber (MMA) with reflectivity below -10 dB ranging from 1.4 to17.31GHz is presented in this article. The MMA has a fractional bandwidth of 170% and a total thickness of 20.8 mm. By analyzing the function of different layers, the MMA can be simplified to the combination of a dual-band (DB) absorber and a single-band (SB) absorber. Layer I consists of a square loop loaded with asymmetric resistors and achieves a DB absorption by exciting high-order modes covering the low-frequency and high-frequency. Layer II is composed of double-ring structures integrated with resistors and realizes an SB absorber in the middle frequency. To understand the physical mechanism in detail, the equivalent circuit model (ECM) is proposed and illustrates a good impedance match within the entire absorption bandwidth with free space. An in-depth analysis has been carried out by illustrating several key dimensions. To verify our idea, a prototype has been designed, fabricated, and measured. Measurement results show an ultra-broadband absorption with the reflectivity below -10 dB ranging from 1.7 to 17.27 GHz under the normal direction, which agrees well with simulation and ECM results.